I am going to post here all newly submitted articles on the arXiv related to superconducting circuits. If your article has been accidentally forgotten, feel free to contact me
17
Dez
2014
Design and experimental study of superconducting left-handed transmission lines with tunable dispersion and improved impedance match
We continue detailed study of microwave properties of a superconducting left-handed tunable CPW transmission line (LHTL). The line consists of a central conductor, loaded with series
of Josephson junctions as fixed inductors; the line is shunted with SQUIDs as tunable inductors. The inductance of the SQUIDs is varied in the range of 0.08-0.5 nH by applying an external dc magnetic field. The circuit is designed to have left- and right-handed transmission bands separated by a variable rejection band. At zero magnetic field, we observed only one pass-band between 8 and 10 GHz within the frequency range of 8-12 GHz. The rejection band is anticipated to appear between 10 GHz and 11 GHz by design, and it has been detected in our previous work. To solve the problem of standing waves and RF leak in measurements of our experimental 20-cell LHTL, we have designed a high-ratio (5-50 Ohm) wideband (8-11 GHz) impedance transformer integrated at the chip, along with improved sample holder. The experimental data are compared with numerical simulations.
Efficient scheme for generation of photonic NOON states in circuit QED
We propose an efficient scheme for generating photonic NOON states of two resonators coupled to a four-level superconducting flux device (coupler). This proposal operates essentially
by employing a technique of a coupler resonantly interacting with two resonators simultaneously. As a consequence, the NOON-state preparation requires only N+1 operational steps and thus is much faster when compared with a recent proposal [Q. P. Su et al., Scientific Reports 4, 3898 (2014)] requiring 2N steps of operation. Moreover, due to the use of only two resonators and a coupler, the experimental setup is much simplified when compared with previous proposals requiring three resonators and two superconducting qubits/qutrits.
Single-photon cavity optomechanics mediated by a quantum two-level system
Coupling electromagnetic waves in a cavity and mechanical vibrations via the radiation pressure of the photons is a promising platform for investigations of quantum mechanical properties
of motion of macroscopic bodies and thereby the limits of quantum mechanics [3,4]. A drawback is that the effect of one photon tends to be tiny, and hence one of the pressing challenges is to substantially increase the interaction strength towards the scale of the cavity damping rate. A novel scenario is to introduce into the setup a quantum two-level system (qubit), which, besides strengthening the coupling, allows for rich physics via strongly enhanced nonlinearities [5-8]. Addressing these issues, here we present a design of cavity optomechanics in the microwave frequency regime involving a Josephson junction qubit. We demonstrate boosting of the radiation pressure interaction energy by six orders of magnitude, allowing to approach the strong coupling regime, where a single quantum of vibrations shifts the cavity frequency by more than its linewidth. We observe nonlinear phenomena at single-photon energies, such as an enhanced damping due to the two-level system. This work opens up nonlinear cavity optomechanics as a plausible tool for the study of quantum properties of motion.
15
Dez
2014
Confining the state of light to a quantum manifold by engineered two-photon loss
Physical systems usually exhibit quantum behavior, such as superpositions and entanglement, only when they are sufficiently decoupled from a lossy environment. Paradoxically, a specially
engineered interaction with the environment can become a resource for the generation and protection of quantum states. This notion can be generalized to the confinement of a system into a manifold of quantum states, consisting of all coherent superpositions of multiple stable steady states. We have experimentally confined the state of a harmonic oscillator to the quantum manifold spanned by two coherent states of opposite phases. In particular, we have observed a Schrodinger cat state spontaneously squeeze out of vacuum, before decaying into a classical mixture. This was accomplished by designing a superconducting microwave resonator whose coupling to a cold bath is dominated by photon pair exchange. This experiment opens new avenues in the fields of nonlinear quantum optics and quantum information, where systems with multi-dimensional steady state manifolds can be used as error corrected logical qubits.
12
Dez
2014
Stückelberg interference in a superconducting qubit under periodic latching modulation
When the transition frequency of a qubit is modulated periodically across an avoided crossing along its energy dispersion curve, tunnelling to the excited state – and consequently
Landau-Zener-St\“uckelberg interference – can occur. The types of modulation studied so far correspond to a continuous evolution of the system along the dispersion curve. Here we introduce a type of modulation called periodic latching, in which the qubit’s free phase evolution is interrupted by sudden switches in the transition frequency. In this case, the conventional Landau-Zener-St\“uckelberg theory becomes inadequate and we develop a novel adiabatic-impulse model for the evolution of the system. We derive the resonance conditions and we identify two regimes: a slow-modulation regime and a fast-modulation regime, in which case the rotating wave approximation (RWA) can be applied to obtain analytical results. The adiabatic-impulse model and the RWA results are compared with those of a full numerical simulation. These theoretical predictions are tested in an experimental setup consisting of a transmon whose flux bias is modulated with a square wave form. A rich spectrum with distinctive features in the slow-modulation and fast-modulation (RWA) regimes is observed and shown to be in very good agreement with the theoretical models. Also, differences with respect to the well known case of sinusoidal modulation are discussed, both theoretically and experimentally.
10
Dez
2014
Simulating dynamical quantum Hall effect with superconducting qubits
We propose an experimental scheme to simulate the dynamical quantum Hall effect and the related interaction-induced topological transition with a superconducting-qubit array. We show
that a one-dimensional Heisenberg model with tunable parameters can be realized in an array of superconducting qubits. The quantized plateaus, which is a feature of the dynamical quantum Hall effect, will emerge in the Berry curvature of the superconducting qubits as a function of the coupling strength between nearest neighbor qubits. We numerically calculate the Berry curvatures of two-, four- and six-qubit arrays, and find that the interaction-induced topological transition can be easily observed with the simplest two-qubit array. Furthermore, we analyze some practical conditions in typical experiments for observing such dynamical quantum Hall effect
09
Dez
2014
Holonomic Quantum Computation via Adiabatic Shortcut
Fast quantum gates based on geometric phases provide a platform for performing robust quantum computation. In particular, non-adiabatic holonomic quantum computation, which involves
non-Abelian geometric phases to achieve universality, has recently been demonstrated in several experiments. Here, we generalize the transitionless quantum driving algorithm to a degenerate Hilbert space, with which we propose a route towards fast holonomic quantum computation. We propose a proof-of-principle experiment in a superconducting circuit architecture to realize our scheme.
08
Dez
2014
Thermal and Residual Excited-State Population in a 3D Transmon Qubit
We present a systematic study of the first excited-state population in a 3D transmon qubit mounted in a dilution refrigerator with a variable temperature. Using a modified version of
the protocol developed by Geerlings et al. [1], we observe the excited-state population to be consistent with a Maxwell-Boltzmann distribution, i.e., a qubit in thermal equilibrium with the refrigerator, over the temperature range 35-150 mK. Below 35 mK, the excited-state population saturates to 0.1%, near the resolution of our measurement. We verified this result using a flux qubit with ten-times stronger coupling to its readout resonator. We conclude that these qubits have effective temperature T_{eff} = 35 mK. Assuming T_{eff} is due solely to hot quasiparticles, the inferred qubit lifetime is 108 us and in plausible agreement with the measured 80 us.
07
Dez
2014
Observation of Dicke Superradiance for Two Artificial Atoms in a Cavity with High Decay Rate
An individual excited two level system decays to its ground state by emitting a single photon in a process known as spontaneous emission. In accordance with quantum theory the probability
of detecting the emitted photon decreases exponentially with the time passed since the excitation of the two level system. In 1954 Dicke first considered the more subtle situation in which two emitters decay in close proximity to each other. He argued that the emission dynamics of a single two level system is altered by the presence of a second one, even if it is in its ground state. Here, we present a close to ideal realization of Dicke’s original two-spin Gedankenexperiment, using a system of two individually controllable superconducting qubits weakly coupled to a microwave cavity with a fast decay rate. The two-emitter case of superradiance is explicitly demonstrated both in time-resolved measurements of the emitted power and by fully reconstructing the density matrix of the emitted field in the photon number basis.
Simulation and Analysis of Superconducting Traveling-Wave Parametric Amplifiers
Superconducting parametric amplifiers have great promise for quantum-limited readout of superconducting qubits and detectors. Until recently, most superconducting parametric amplifiers
had been based on resonant structures, limiting their bandwidth and dynamic range. Broadband traveling-wave parametric amplifiers based both on the nonlinear kinetic inductance of superconducting thin films and on Josephson junctions are in development. By modifying the dispersion property of the amplifier circuit, referred to as dispersion engineering, the gain can be greatly enhanced and the size can be reduced. We present two theoretical frameworks for analyzing and understanding such parametric amplifiers: (1) generalized coupled-mode equations and (2) a finite difference time domain (FDTD) model combined with a small signal analysis. We show how these analytical and numerical tools may be used to understand device performance.